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SN74LV4T08-Q1: SN74LV4T08-Q1 Maximum Input Transition Time

Craig MacKinnon
Prodigy 90 points
Part Number: SN74LV4T08-Q1
Other Parts Discussed in Thread: SN74AHC1G14, SN74LVC1G14

Tool/software:

Hi TI E2E

I have need of a circuit that requires a delay time of approximately one second. In order to prevent any issues with shoot through on the logic gates, I have chosen to use the SN74LV4T08-Q1 with Schmitt triggered inputs however, I've discovered that the maximum transition time for a 1.8V design is about 36 ns. This design is then clearly in violation of that specification. I have read on this E2E forum that tolerance for slow input rise times is a function the amount of hysteresis that device has on its input with larger hysteresis making for an input more tolerant of a slow transitioning input signal. I couldn't find any information in the datasheet regarding the amount of hysteresis on the SN74LV inputs. If the SN74LV is in violation of the rise time spec, is there an alternate logic family that will meet this requirement?

Cheers,

Craig M

  • +1
    Guru 314780 points

    The SN74LV4T08-Q1 device does not have Schmitt-trigger inputs. If you do not need voltage translation, use something like the SN74LVC1G14/SN74AHC1G14.

    Schmitt-trigger inputs allow slower edges, but do not reduce the shoot-through current. If you care about the power consumption during switch, use a comparator instead.

  • 0
    Prodigy 90 points

    Thanks for the feedback! BTW, check the datasheet because it does state in the Description that the inputs are Schmitt triggered although nothing else in the document indicates this.  

  • 0 in reply to Craig MacKinnon
    Guru 314780 points

    Devices with very small propagation delays always are designed with a little bit of hysteresis to counteract the inductance of the pins and bond wires, but the guaranteed amount of hysteresis visible to your circuit is zero.